There is a real and growing need for cost/effective solar conversion systems with combined modes of operation and effective year-round solar energy utilization. Individual home-owners are generally reluctant to proceed with the installation of solar energy units and systems when faced with retrofitting problems and doubts about the year round effective operation aimed at reducing ever increasing home utility bills.
Because of the intermittant nature of solar energy availability any solar conversion system must be very effective in providing energy for useful levels of household heating, cooling and electrical power plus domestic hot water on a nearly year round basis.
Most of the solar energy conversion units and systems now being marketed are of the flat plate collector type which generate hot water which can supply a portion of the household heating needs plus domestic hot water. These systems are usually expensive for retrofitted installations in existing homes and buildings, particularly when the system is only effective during the summer months in most latitudes of the U.S. Not much attention has been directed toward the generation of electric power from these systems during the summer months because their operating temperatures are well below the practical temperature levels for effective heat engine operation.
There are certain fundamental principles in solar energy conversion hardware which are both rigid and inherent in their application to each type of house or building configuration--ie:--flat plate collectors for both flat and gabled rooftops are used for both domestic hot water and space heating. The flat plate collectors are generally rigid in design configuration, but adaptable to most any type of building rooftop providing that a southerly exposure is evident for one rooftop surface.
On the other hand, the highly effective parabolic/hyperbolic concentrating solar converters are flexible in design and multiple modes of operation, but are usually rigid in their application to buildings with flat rooftops only.
This installation adaptability situation for these types of solar conversion components leads to the application of passive and active flat plate collectors for home installations, while the more effective, higher temperature solar concentrators are natural capital equipment installations for apartment houses and flat rooftop industrial buildings and plants.
There are ways, however, of applying the more effective solar concentrators for individual homes in cases where the southerly side of the home as the back yard has a generally unobstructed exposure to the sun's excursion during the day. The high temperature linear solar concentrators can be mounted on raised structures such as carports, greenhouses or other similar secondary structures on the property. Another option is that the solar conversion system be arranged as a portable modular unit which may be placed at the most suitable location on the property. It is critical to the effective operation of these solar concentrators that they are not shadowed by adjacent trees and structures of any kind during the major portion of each solar day.
Since the linear concentrators--(parabolic and hyperbolic) solar sections are flexible in design and modes of operation there are major advantages to be gained from combining both hot water energy and hot air energy for winter time space heating, plus the generation of electrical power for summer time power/air conditioning from a single system installation for most types of building configurations.
Multi-mode solar conversion will have significant economic value for both large apartment houses, condominiums, and industrial plants plus some individual homes, since the pay-back time would be considerably shorter and far higher power generation levels would be achieved over nearly all seasons of the year. If solar energy utilization is to have any real impact on easing the oncoming energy shortfall, individual homeowners must be convinced that they are going to substantially reduce their total energy costs with the installation of a proven, integrated solar conversion system. The present D.O.E. policy of funding and constructing hugh centralized solar and wind energy conversion sites is both wasteful and cost/ineffective while serving only the power needs of specific localities through the influence of local politicians, and power utilities.
If the present D.O.E. policy of supporting hugh centralized alternate energy sites is continued then this government department will rapidly become a part of the overall energy problem rather than producing practical solutions to the many problems. Any alternate energy policy which does not stress the wide participation of the general public and individual homeowners will not solve the oncoming energy shortfall, since hugh centralized energy sites provide only a drop in the power bucket in the U.S. and serve the power utilities without any surety of reduced power rates to the general public.
Because the state-of-the-art in silicon photovoltaic cells including the latest amorphous silicon film strips continues to be discouraging from a cost standpoint, some sort of multi-mode, convertible solar concentration approach appears to be more worthwhile because one system installation can provide effective home heating service and electric power on a nearly year round basis. No matter how efficient silicon solar cells become, while costs continue to be reduced, they can never come close to matching the cost/effectiveness of both direct solar hot air and hot water heating for individual homes and commercial buildings.
Although the heat engine and electrical generator means for a multi-mode solar conversion system are relatively expensive components there is a basic cost advantage in utilizing existing winter time solar conversion concentrators for summer-time solar energy conversion.
While the silicon solar cell industry and the D.O.E. have made optimistic cost projections for improved silicon solar cells it is becoming apparent that future cost advantages will be confined to relatively large volume purchases for large array installations in the 5 to 50 kilowatt range, which will serve large commercial sites, apartment houses and condominiums. Solar cell costs will probably be excessively high in the fractional kilowatt and up to 5 kilowatt range, which would be the range for the individual homeowners. Small sub-fractional K.W. arrays of silicon solar cells will be necessary for a multi-mode solar conversion system to power the fans within the hot-air ducts of the system. There is a natural advantage to using silicon solar cells for the air moving power means within the hot air ducts, since the fans will operate only when there is sufficient solar insolation on the concentrator sections. This portion of the system arrangement will essentially be automatic in operation and the solar cell array may remain stationary and remotely located from the solar concentrator sections.
No more than three fan/motors will be required for the average home-sized solar conversion system and therefore the silicon solar cell array to power the fan motors may be limit ed to about 1/8 KW, or 125 watts, max., at about 6 to 12 volts. The fan motors within the air ducts will be located at the duct entrance, exit and mid-length of the hot air duct loop.
Several major problems exist which must be resolved before a cost/effective summer time solar conversion mode of operation can be achieved. The design of the closed Rankine cycle engine loop is critical from both the performance and cost standpoints, and carefully considered cost/effectiveness tradeoffs must be made for each component of the heat engine loop. The water-to-steam heating portion of the engine cycle offers no major problems, but the condensation phase presents several difficulties since a large effective heat transfer surface area is necessary with corresponding high fabrication and/or tooling costs.
Both the rotary steam expander (engine) and the return pump may be off-the-shelf components, so that these do not present major performance and cost problems. The cooling/condensation phase of the heat engine cycle impose major obstacles, as previously stated, and must effectively and rapidly cool the expended steam flow from the expander without excessive steam flow impedance. There are steam expansion/condensing components that can meet the necessary performance requirements, but these must receive further development effort before being a cost-acceptable heat engine component for solar energy conversion applications.
Similar problems exist for the Freon vaporization wheel drive which is based on the publicized Minto Wheel concept. There is considerable room for improvement in the basic Freon vaporization/condensation wheel drive unit which include the use of hot and cold air as the heating and cooling means instead of hot water/cold air is projected as a further advantage in increasing the wheel speed and therefore efficiency.
Since the Freon vaporization wheel unit is relatively large in size from six to eight feet in diameter it may be integrated into the tri-mode solar conversion portable module, as a removable component for servicing and/or replacement.
Because of the overall complexity of the improved Freon vaporization wheel drive it must be considered as an optional power means which is secondary to the basic Rankine cycle-heat engine sub-system. The Rankine cycle heat engine may be used for air conditioning electrical power or for any household electrical power requirement. If the Freon vaporization wheel drive is omitted as a sub-system, then the hot air duct will remain disconnected and left vented at both ends of the duct loop and connected only for winter time useage.
The primary A/C, air conditioning means for the convertible modular tri-mode solar conversion system may be the conventional and simple absorption/chiller cycle which is ideal for relatively large household A/C cooling loads. Since the closed Rankine cycle heat engine sub-system and the absorption chiller cycle must remain as separate sub-systems for maximum effectiveness, a second straight piping line must be located directly under and in tangent contact with the primary focal piping line. The focal piping will be exposed to higher solar temperatures for efficient Rankine cycle operation, while the slightly lower temperature of the lower piping will produce an adequate steam flow for the absorption chiller cycle.
Full tri-mode solar conversion is provided with or without the Freon vaporization wheel drive, but the inclusion of this sub-system would provide the highest possible solar conversion effectiveness from a single portable modular system without any operating penality imposed on any of the other sub-systems.